1. Field of the Invention
Embodiments of the invention relate to polysilicon fuses and semiconductor devices having polysilicon fuses.
2. Related Art
A polysilicon fuse is used as a trimming device for carrying out trimming that finely adjusts the value of an element such as the resistance value of a resistor in a semiconductor circuit such as a semiconductor integrated circuit formed in a semiconductor substrate. For example, a plurality of resistors connected in parallel with one another are provided in a semiconductor integrated circuit, to each of which resistors a polysilicon fuse as a trimming device is connected. Adjustment of the resistance value is carried out by blowing the polysilicon fuses with the use of a laser irradiation method or a voltage application method. In particular, the voltage application method is a simple and easy method that can be carried out together with a wafer characteristics test carried out after the fabrication process of the semiconductor integrated circuit has been completed.
FIG. 16A is a plan view showing the structure of the principal part of a related polysilicon fuse 600. FIG. 16B is a cross sectional view cut along line X-X of FIG. 16A.
The principal part of a related polysilicon fuse 600 is provided with a narrowed portion 51 of narrow width polysilicon, two electrode portions 52 of wide width polysilicon and two metal electrodes 54 each connected to the electrode portion 52 through a via hole 53. The narrowed portion 51 and electrode portions 52 are formed on an insulating film 55 formed by the LOCOS (Local Oxidation of Silicon) process etc. Between the electrode portion 52 and the metal electrode 54, an interlayer dielectric 56 (HTO (High Temperature Oxide) film etc.) having via holes 53 is arranged with a surface protecting film 57 provided to cover the metal electrodes 54. For the polysilicon forming the polysilicon fuse 600, doped polysilicon or ion-implanted polysilicon is used, the specific resistance of which is adjusted by adjusting an amount of doping or an amount of implanted ions. In the following, blowing of a polysilicon fuse by the voltage application method will be explained.
Application of a voltage to the metal electrodes 54 of the polysilicon fuse 600 causes a current to flow in the narrowed portion 51, which is heated to fuse. At this time, the polysilicon of the electrode portions 52 close to the narrowed portion 51 also fuses. The polysilicon fused at the narrowed portion 51 and at each of the electrode portions 52 is attracted to the side of each of the electrode portions 52 by the surface tension of the fused polysilicon to be severed. At this time, at the narrowed portion 51, the fused polysilicon is finely severed. With the fused polysilicon severed (blown) at the narrowed portion 51, the polysilicon fuse 600 is brought into an electrically insulated state. The severed polysilicon fuse 600 reduces the number of resistors formed in parallel in the semiconductor circuit for adjusting the circuit characteristics, by which the resistance value in the semiconductor circuit is adjusted.
Next, related methods disclosed in patent documents will be explained, each of which methods is for stably blowing an electric fuse by using the voltage application method.
In Japanese Patent Application Publication No. JP-A-2003-258104, a method is disclosed, in which an opening is provided on the side of the fuse blowing section of a polycrystalline silicon fuse and an insulating film on the side wall side of the fuse is made to rupture toward the opening without affecting an insulating film on the fuse.
In Japanese Patent Application Publication No. JP-A-2010-251499, a method is disclosed in which a gap is provided on the side surface of the fuse blowing section of an electric fuse. In Japanese Patent No. 4,164,054, a method is disclosed in which the stress in an interlayer dielectric covering a polysilicon fuse is relaxed for preventing faulty breaking of the polysilicon fuse.
In Japanese Patent Application Publication No. JP-A-2006-73947, it is disclosed that an electric fuse structure has an upper layer electric fuse and a lower layer electric fuse arranged vertically. By applying a bias voltage to each of the upper layer electric fuse and lower layer electric fuse, the fuses can be blown. Furthermore, one end of the upper layer electric fuse and one end of the lower layer electric fuse are commonly connected to enable the potential at the one end of the upper layer electric fuse and the potential at the one end of the lower layer electric fuse to be common to them easily. The electric fuse is used for switching memory arrays.
A semiconductor device having the polysilicon fuse described in the foregoing is required to have a high reliability. In such a semiconductor device, however, when the device is operated at a high temperature, residues of polysilicon finely divided at blowing are connected to one another by a migration phenomenon to cause a leak current to flow to make the polysilicon fuse impossible to stably maintain an electrically insulated state. This causes the resistance value of a resistor in a semiconductor circuit to deviate from an optimum value to affect the circuit characteristics of the semiconductor device.
FIG. 17 is an enlarged cross sectional view showing the narrowed portion 51 after blowing of the polysilicon fuse 600. When residues 58 in the space left after the blowing of the polysilicon of the narrowed portion 51 are joined to the polysilicon of the electrode portions 52 by a migration phenomenon as is shown by dotted lines, a leak current flows. The action of the migration phenomenon is accelerated at a high temperature (at operation) to change circuit characteristics of the semiconductor device in operation to lower the reliability of the semiconductor device.